Summary: After decades of stagnation, the size of the middle class in Latin America and the Caribbean recently expanded by 50 percent from 103 million people in 2003 to 152 million (or 30 percent of the continent's population) in 2009. Over the same period, as household incomes grew and inequality edged downward in most countries, the proportion of people in poverty fell markedly: from 44 percent to 30 percent. As a result, the middle class and the poor now account for roughly the same share of Latin America's population. This is in stark contrast to the situation prevailing (for a long period) until about 10 years ago, when the share of the poor hovered around 2.5 times that of the middle class. This study investigates the nature, determinants, and possible consequences of this remarkable process of social transformation. Such large changes in the size and composition of social classes must, by definition, imply substantial economic mobility of some form. A large number of people who were poor in the late 1990s are now no longer poor. Others who were not yet middle class have now joined its ranks. But social and economic mobility does not mean the same thing to different people or in different contexts. This report discusses the relevant concepts and documents the facts about mobility in Latin America and the Caribbean over the past two decades, both within and between generations. In addition, it investigates the rise of the Latin American middle class over the past 10-15 years and explores the size, nature, and composition of this pivotal new social group. More speculatively, it also asks how the rising middle class may reshape the region's social contract.